Autoconf has recently seen some effort to robustify configure tests
against picky GCC warning settings. There is quite a way to go yet, but
it may still be useful to move the OpenMPI configury in that direction,
too. So below is a patch that picks some low-hanging fruit on the way
to make
../ompi-trunk/configure -C CFLAGS='-W -Wall -Werror' \
CXXFLAGS='-W -Wall -Werror'

work better with GCC. Note that currently, the above will still produce
some wrong results, or may even fail one some systems; so it's really
just a first step in that direction.

Also please double-check the changes to Windows specific files -- I did
not test them in any way.

As a side effect, this patch fixes one typo in a comment, and one
underquoted pair of brackets that would make code from c_weak_symbols.m4
end up as
int main(int argc, char *argv);